调用 QGraphicsScene::advance() 后，我的项目没有重绘

Question

I was trying to do a simple animation on QGrapichScene. I implemented void QGraphicsItem::advance(int) in class, that inherites QGraphicsItem, but after calling advance() my item not redrawn. In colliding mice example it works. What have I done wrong?

Here is my code:

widget.h:

class Widget : public QWidget
{
    Q_OBJECT

private:
    QGraphicsScene *scene;
    QGraphicsView *view;
    QHBoxLayout *layout;
    QTimer t;

public:
    Widget(QWidget *parent = 0);
    ~Widget();
};

widget.cpp:

Widget::Widget(QWidget *parent) : QWidget(parent)
{
    layout = new QHBoxLayout(this);

    view = new QGraphicsView(this);
    scene = new QGraphicsScene(0, 0, 400, 400, view);

    scene->addItem(new MyItem());

    view->setScene(scene);
    layout->addWidget(view);

    setLayout(layout);

    connect(&t, SIGNAL(timeout()), scene, SLOT(advance()));

    t.start(100);
}

Widget::~Widget()
{

}

my_item.h:

class MyItem : public QGraphicsItem
{
private:
    QRect bRect;

    enum directon { left, right };

    directon currentDir;

protected:
    virtual void paint(QPainter *painter, const QStyleOptionGraphicsItem *option, QWidget *widget);

public:
    MyItem(int w = 20);

    virtual void advance(int phase);
    virtual QRectF boundingRect() const
    { return QRectF(bRect); }
};

my_item.cpp:

MyItem::MyItem(int w)
{
    currentDir = right;
    bRect = QRect(0, 0, w, w);
}

void MyItem::paint(QPainter *painter, const QStyleOptionGraphicsItem *option, QWidget *widget)
{
    qDebug() << "In void MyItem::paint(QPainter*, "
                "const QStyleOptionGraphicsItem*, "
                "QWidget*)";

    painter->fillRect(bRect, Qt::red);
}

void MyItem::advance(int phase)
{
    qDebug() << "In void MyItem::advance(int);"
             << "Phase =" << phase;
    if(!phase) 
        return;
    // Than move item to new positon...
}

Answer 1

It's on you to inform the graphics scene that your item's contents have changed. You have to call update() at the end of advance() . If you're simply moving the item, without changing its contents, then you don't need to call update() of course - the scene will detect such changes automatically.

Answer 2

I found something strange:

connect(&timer, SIGNAL(timeout()), scene, SLOT(advance()));

The slot need to combined after the Widget is shown, or the slot will never been called!

As the Qt example shown, you can combine it in the main function.